Tc 9-524 fundamentals of machine tools

NOTE: 
It is good practice always to index clockwise on
the plate to eliminate backlash.
Plain Indexing
The following principles apply to basic indexing of
workpieces:
Suppose it is desired to mill a project with eight equally
spaced teeth. Since 40 turns of the index crank will turn the
spindle one full turn, l/8th of 40 or 5 turns of the crank after
each cut will space the gear for 8 teeth, If it is desired to
space equally for 10 teeth, 1/10 of 40 or 4 turns would
produce the correct spacing.
The construction of some index heads permits the worm to
be disengaged from the worm wheel, making possible a
quicker method of indexing called direct indexing. The index
head is provided with a knob which, when turned through
part of a revolution, operates an eccentric and disengages the
worm.
Direct indexing is accomplished by an additional index
plate fastened to the index head spindle. A stationary plunger
in the index head fits the holes in this index plate. By
moving this plate by hand to index directly, the spindle and
the workpiece rotate an equal distance. Direct index plates
usually have 24 holes and offer a quick means of milling
squares, hexagons, taps, and so forth.
Any number of
divisions which is a factor of 24 can be indexed quickly and
conveniently by the direct indexing method.
Differential Indexing
Sometimes, a number of divisions is required which cannot
be obtained by simple indexing with the index plates
regularly supplied. To obtain these divisions, a differential
index head is used. The index crank is connected to the
wormshaft by a train of gears instead of a direct coupling as
with simple indexing. The selection of these gears involves
calculations similar to those used in calculating change gear
ratio for lathe thread cutting.